Now it seems the answer may be a little more mundane: the biggest known hole in the universe.

The cold spot appears in maps of the cosmic microwave background (CMB), the earliest light emitted in the universe. Temperature variations in the light show up as a mottled pattern in the maps, which can be explained if quantum fluctuations at the universe's birth were stretched out by a brief but spectacular cosmic growth spurt known as inflation.

But some features in the maps don't fit into the leading models of inflation. For example, the relatively even pattern of the CMB is marred by an unusually large cold region. Scientists have struggled to explain it, suggesting a number of ideas that require exotic physics or even evidence for a multiverse.

A much simpler explanation is that the cold spot is caused by a giant void in the universe. The cosmos consists of a web of bright galaxies and clusters surrounded by dark pockets that contain little matter. Radiation loses energy when it crosses these empty regions, so a large void could cause a cold spot in our CMB maps. But most surveys haven't looked at a wide enough region of the sky to be able to find such a void relatively close to Earth. One study that claimed to have discovered one in 2007 was later disputed.

István Szapudi at the University of Hawaii in Honolulu and his colleagues analysed an all-sky survey made by NASA's WISE satellite to conduct their own hunt for a giant void. In May, they reported finding one about 2.8 billion light years away, in the direction of the cold spot. It stretches some 1.8 billion light years across, making it about twice the size of the previous largest known void, says Szapudi.

Now the team has studied the properties of the so-called supervoid, including its alignment with the cold spot and its apparent depth. A number of techniques all yielded similar results, which the team says bolsters the case linking the void to the cold spot (arxiv.org/abs/1406.3622).

"This would be the simplest explanation requiring no exotic physics," says Szapudi. He adds that similarly simple causes may lie at the heart of other CMB mysteries, such as temperature differences that seem to be aligned along a preferred direction, dubbed the "axis of evil".

Dragan Huterer at the University of Michigan in Ann Arbor questioned the 2007 claim, but he thinks Szapudi's team makes a good case for their void. "This is a very exciting finding," he says.

This article appeared in print under the headline "Biggest hole in universe solves cosmic cold case"

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